SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Migrates the shader generator off the use of a global array, eliminating
the use of some global state. This also allows us to move the shader
generation over to using fmt in a subsequent change.

Improve bookkeeping around formats. Hopefully make code less confusing.

- Rename TlutFormat -> TLUTFormat to follow conventions.
- Use enum classes to prevent using a Texture format where an EFB Copy format
  is expected or vice-versa.
- Use common EFBCopyFormat names regardless of depth and YUV configurations.

Currently, we use the alpha channel from the EFB even if the current
format does not include an alpha channel. Now, the alpha channel is set
to 1 if the format does not have an alpha channel, as well as truncating
to 5/6 bits per channel. This matches the EFB-to-texture behavior.

Allows for forward declarations in most places, which prevents dumping
unrelated VideoCommon.h contents directly into headers.